In IMRT a target region like a tumor region is illuminated from different directions by using different illumination distributions, wherein these illumination distributions are adapted such that, after the target region has been illuminated by the different illumination distributions in the different directions, the target region has received the most radiation dose, whereas the surrounding of the target region has received a radiation dose being as small as possible. The illumination distributions are generally determined in at least two steps. In a first step for each illumination direction a fluence map is calculated under radiation dose constraints, which substantially defines which region should receive which radiation dose, and in a second step for each illumination direction the respective fluence map is transformed into a sequence of illumination distributions realizable by a radiation device used for performing the IMRT. This transformation of the calculated fluence map to the sequence of realizable illumination distributions modifies the radiation doses, which will finally be received by the different regions, and therefore leads to a reduced quality of the IMRT.